1. Field of the Invention
The present invention relates generally to an air fuel mixer for the combustor of a gas turbine engine and, more particularly, to a dual fuel mixer for the combustor of a gas turbine engine which uniformly mixes either liquid and/or gaseous fuel with air so as to reduce NOx formed by the ignition of the fuel/air mixture.
2. Description of Related Art
Air pollution concerns worldwide have led to stricter emissions standards requiring significant reductions in gas turbine pollutant emissions, especially for industrial and power generation applications. Nitrogen Oxides (NOx), which are a precursor to atmospheric pollution, are generally formed in the high temperature regions of the gas turbine combustor by direct oxidation of atmospheric nitrogen with oxygen. Reductions in gas turbine emissions of NOx have been obtained by the reduction of flame temperatures in the combustor, such as through the injection of high purity water or steam in the combustor. Additionally, exhaust gas emissions have been reduced through measures such as selective catalytic reduction. While both the wet techniques (water/steam injection) and selective catalytic reduction have proven themselves in the field, both of these techniques require extensive use of ancillary equipment. Obviously, this drives the cost of energy production higher. Other techniques for the reduction of gas turbine emissions include "rich burnt quick quench, lean burn" and "lean premix" combustion, where the fuel is burned at a lower temperature.
In a typical aero-derivative industrial gas turbine engine, fuel is burned in an annular combustor. The fuel is metered and injected into the combustor by means of multiple nozzles along with combustion air having a designated amount of swirl. Until recently, no particular care has been exercised in the prior art in the design of the nozzle or the dome end of the combustor to mix the fuel and air uniformly to reduce the flame temperatures. Accordingly, non-uniformity of the air/fuel mixture causes the flame to be locally hotter, leading to significantly enhanced production of NOx.
In the typical aircraft gas turbine engine, flame stability and engine operability dominate combustor design requirements. This has in general resulted in combustor designs with the combustion at the dome end of the combustor proceeding at the highest possible temperatures at stoichiometric conditions. This, in turn, leads to large quantities of NOx being formed in such gas turbine combustors since it has been of secondary importance.
While premixing ducts in the prior art have been utilized in lean burning designs, they have been found to be unsatisfactory due to flashback and auto-ignition considerations for modem gas turbine applications. Flashback involves the flame of the combustor being drawn back into the mixing section, which is most often caused by a backflow from the combustor due to compressor instability and transient flows. Auto-ignition of the fuel/air mixture can occur within the premixing duct if the velocity of the air flow is not fast enough, i.e., where there is a local region of high residence time. Flashback and auto-ignition have become serious considerations in the design of mixers for aero-derivative engines due to increased pressure ratios and operating temperatures. Since one desired application of the present invention is for the LM6000 gas turbine engine, which is the aero-derivative of General Electric's CF6-80C2engine, these considerations are of primary significance.
U.S. Pat. No. 5,251,447 to Joshi et al., which is owned by the assignee of the present invention, describes an air fuel mixer in which gaseous fuel is injected into the mixing duct thereof by means of passages in the vanes of an outer swirler. This concept was also utilized in U.S. Pat. No. 5,351,477 to Joshi et al, which is also owned by the assignee of the present invention, along with a separate manifold and passage through a hub between the outer and inner swirlers to provide dual fuel (gaseous and/or liquid) capability to the air fuel mixer. It has further been disclosed in three related applications, each entitled "Dual Fuel Mixer For Gas Turbine Combustor" and having Serial Nos. 08/581,813, 08/581,817, and 08/581,818, that liquid fuel alternatively may be provided radially to the mixing duct via certain passage configurations in a centerbody of the air fuel mixer. In each of these air fuel mixer designs, a centerbody is provided with the mixing duct thereof. It has been found that complex air flow may cause recirculation zones to form within the mixing duct which create a greater probability of flashback or autoignition along the centerbody.
Accordingly, it would be desirable for an air fuel mixer to be developed for the combustor of a gas turbine engine which has the capability of mixing gaseous and/or liquid fuel therein without the flashback/autoignition problems associated with having a centerbody in the mixing duct thereof.